Introduction

During the past decade, the use of nanotechnology has become more prevalent among the various biomedical applications, specially bioimaging, drug delivery, antibacterial activities, and this is due to the high surface to volume ratio and remarkable chemical reaction of nanomaterials (Ghosal and Sarkar 2018; Hai 2018a, b; Nakhjavani 2017; Rajeshkumar and Naik 2018). Several nanomaterials, notably magnetite nanoparticles (MNPs), with their great properties, such as high magnetism, low toxicity, and having a remarkable biocompatible surface, have attracted enormous worldwide researchers’ attention (Gao 2016; Powell 2020). Magnetic nanomaterials, e.g., superparamagnetic iron oxide nanoparticles (SPIONs), are prime candidates for a wide range of applications in biomedical sciences and technologies such as magnetite resonance imaging (MRI) contrast agent, cancer therapy, drug delivery, cell labeling, and tracking (Yang 2018; Huang 2016; Ahmadi and Gu 2012). According to the size and shape distribution of the MNPs, tremendous methods of synthesis have been developed, which the co-precipitation approach is one of the most convenient synthesis methods for biomedical applications (Dubey and Kain 2018; Kulkarni 2014). Besides, nanofluids' novel characteristics can be used in a wide range of applications, especially heat transfer, electronics, biomedicine, and food (Sarfaraz 2018, 2019a; b). Besides, nanofluids exhibit enhanced thermal conductivity and convective heat transfer as compared with other kinds of fluids (Shahsavar 2019; Akbari 2016; Bahmani 2018; Nazari 2020). Among all nanofluids, ferrofluids containing MNPs exhibit high nanoparticle stability with biomedical applications.

Particle aggregation due to the long-range Vander Waal's forces and their high inclination to oxidize are the bare MNPs' properties' unwilling characteristics. One of the most conventional and economic approaches to prevent these spontaneous features and improve the dispersion behavior in solutions of pristine MNPs is a surface modification (Farjadian 2016; Liu 2017). Amino acids are desirable agents among the surfactants as the fine ligands for nano-sized magnetite particle coating (Xia 2016). LCysteine (LCys) is a biocompatible, natural amino acid with the formula of [HO2CCH(NH2)CH2SH]. This amino acid has three functional groups, including carboxylic (-COOH), amino (-NH2), and thiol (-SH). Therefore, LCysteine with desirable binding properties could prevent the oxidation of MNPs and convert them to particles with significant solubility and stability in organic solutions and provides better bio-compatibility behavior for the final products (Bagbi 2017; Ahmadi 2011).

Recently, graphene, a two-dimensional (2D) carbon allotropy, has drawn considerable attention because of its superiority in novel electrical and optical properties compared to other materials. Graphene is the one-atom-thick sheet of sp2 hybridized carbon atoms, which is arranged in a honeycomb-packed structure (Fang et al. 2015; Naumis 2016). Many researchers have studied graphene quantum dots (GQDs) as a zero-dimensional (0D) carbon material with a size of less than 100 nm in all dimensions. Owing to the edge effect and quantum confinement effect (QCE), GQDs possess novel and unique properties, which can never be seen in pristine materials (Hai 2018a, b; Zhang et al. 2018; Bacon 2014; Kim 2012). In comparison with some semiconductors, GQDs exhibit outstanding properties, such as high photostability against photoblinking and photobleaching, low toxicity, eco-friendly, and biocompatibility. GQDs have shown excellent performance in different areas of biological applications, especially in biosensing for detection of various moieties like inorganic ions, small organic molecules, and bio-macromolecules, as well as bioimaging for monitoring organs, tissues, and living cell, such as cell imaging, targeted imaging, diagnostic imaging, and in vivo imaging (Zhang and Ding 2018; Zheng 2015; Russo 2016; Hai et al. 2018a, b). The properties of the GQDs are directly correlated to their physical features, such as its size, shape, and edge configuration, which are affected by their synthesis methods (Gu 2016; Wang 2015).

Herein, this work reports a facile and novel method for synthesizing Fe3O4–LCysteine–GQDs multifunctional nanocomposite through two steps. While several research papers have been used different methods to obtain Fe3O4-GQDs nanostructure, we report a facile, two-step route to synthesize magnetic GQDs nanostructure. We also utilize LCysteine to prevent MNPs from oxidation and activate its surface due to its functional groups, which act as a linker between magnetite nanoparticles and GQDs. However, several research works have been done on the individual elements that we used in the synthesized nanostructure, we produce the multifunctional hybrid nanocomposite with high magnetic property and stable optical characteristics that can be used in a wide range of biomedical application, especially MRI contrast agents and bioimaging.

Materials and methods

Chemical reagents

All chemical reactants and reagents used in this research were of analytical grade and were used as received without further purification. Iron (II) chloride tetrahydrate (FeCl2.4H2O, Merck, CAS number: 13478-10-9, Iron (III) chloride hexahydrate, (FeCl3.6H2O, Merck, CAS number:10025-77-1), ammonia solution 25% (NH4OH, Merck, CAS number: 7664-41-7), Hydrochloric acid (HCl, Merck, CAS number: 7647-01-0), citric acid (C6H8O7, Merck, CAS number: 77-92-9), sodium hydroxide (NaOH, Merck, CAS number: 1310–73-2), and LCysteine (C3H7NO2S, BioChem, CAS number: 52–90-4) were used as the synthesizing reagents.

Synthesis of \({\text{L}}_{{{\text{Cysteine}}}}\) functionalized MNPs

LCysteine functionalized Fe3O4 nanoparticles were synthesized using the co-precipitation method. The process describes as follows: first, the MNPs were obtained by solving 4 mmol of FeCl3·6H2O and 2 mmol of FeCl2.4H2O (molar ration 2:1), in 20 and 5 mL of the 2 M HCl, respectively. The obtained solutions were mixed and stirred in 40 °C for 15 min under N2 atmosphere. As to obtain suitable pH in the range of 11–12 (Ahmadi 2011), 35 mL of NH4OH 25% solution was dropwise added to the solution with the rate of 15 \({ }{\raise0.7ex\hbox{${{\text{drops}}}$} \!\mathord{\left/ {\vphantom {{{\text{drops}}} {{\text{min}}}}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{${{\text{min}}}$}}\) to increase the pH of the solution. The black precipitates were collected from the water by magnetic decantation then washed 3 times with distilled water and acetone to remove impurities. The general reaction can be written as

$${\text{Fe}}^{{2 + }} + 2{\text{Fe}}^{{3 + }} + 8{\text{OH}}^{ - } ~{\text{~Fe}}_{3} {\text{O}}_{4} + 4{\text{H}}_{2} {\text{O}}.$$
(1)

Afterward, 0.370 gr MNPs with 10 mL water ultrasound for about 30 min. Immediately, 1 mmol LCysteine (0.121 gr), dissolved in 5 mL distilled water for 10 min by ultrasonication the mixture at 25 °C, was dropwise added to the MNPs solution. Subsequently, the products were stirred overnight. The synthesized nanoparticles should be dried at 50 °C at the oven for 24 h (Fig. 1a).

Synthesis of GQDs

Pristine GQDs was fabricated by pyrolyzing citric acid. In a typical run of GQDs preparation, 2 gr acid citric was put in the 5 mL beaker and heated for 30 min at 200 °C to turn the colorless citric acid powder to the pale orange liquid. Obtaining this temperature is essential in this process, because producing GQDs is not feasible at temperatures below 200 °C. The obtained liquid was mixed with the 100 mL of 0.25 M NaOH, and the solution was continuously stirring for about 5 min. Finally, the resultant solution was neutralized to pH = 7 by adding drops of 2 M HCl and cooling to room temperature. This solution should be stored in a dark place and temperature of about 15 °C (Fig. 1b).

Synthesis of core–shell structure of MNPs with GQDs

First, 0.268 gr of functionalized MNPs should be dispersed in 10 mL distilled water and stirred continuously for 15 min and subsequently, 15 mL GQDs were gradually added to the mixture and stirred for 24 h. After the one-day stirring of the solution, the resultant mixture was dried at 50 °C for 24 h. Synthesis of LCysteine coated magnetic nanoparticles with GQDs leads to the formation of Fe3O4–LCysteine–GQDs nanocomposite as a dual-functional nanomaterial (Fig. 1c).

Fig. 1
figure 1

Schematic representation of the synthesis approach of a GQDs by hydrothermal method and b Lcysteine functionalized Fe3O4 nanoparticles by co-precipitation route to obtain c Fe3O4-Lcysteine-GQDs nanostructure

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Analyses and characterizations

The X-ray diffraction patterns of powder samples were obtained by a PANalytical X’pert PRO MPD diffractometer using Cu-Ka radiation with a wavelength of 1.5406 Å. The Fourier-transform infrared (FT-IR) spectra were collected in transmission mode with a Thermo Nicolet AVATAR 360 FT-IR spectrometer in the spectral range of 400–4000 cm−1 at room temperature. FT-IR powder samples were dried at 50 °C before the fabrication of the KBr pellet. X-ray photoelectron (XPS) spectroscopy was carried out with a monochromatic Al Ka X-ray source using chamber pressures of about 10–8 Pa. Conventional transmission electron microscopy (TEM) observations were done using a Philips CM300 microscope to determine the average particle size and morphology of the powder on an accelerating voltage of 200 kV. The surface morphology of the sample was observed by the Tescan MIRA3 scanning electron (SEM) microscope. In addition, an MDK-VSM system was used for magnetic characterizations. The optical properties were evaluated using the Cary Eclipse photoluminescence (PL) spectrometer.

Results and discussion

X-ray powder diffraction (XRD) measurement

The XRD patterns of synthesized bare Fe3O4, Fe3O4–LCysteine, and Fe3O4–LCysteine–GQDs nanostructures in comparison with the standard pattern of Fe3O4 nanoparticles (JCPDS data, card no. 96-900-5815), and Fe2O3 nanoparticles (JCPDS data, card no. 96-101-1241) are presented in Fig. 2. According to the standard pattern of magnetite nanoparticles, which is shown in Fig. 2a, the well-match pattern can be observed in Fig. 2c and confirms the inverse spinel and face-centered cubic (FCC) structure of synthesized magnetite particles which is belonging to the Fe3O4 nanoparticles. The average crystalline size of the Fe3O4 nanoparticles was determined using the Debye–Scherrer formula and full-width at half-maximum (FWHM) of the three most intense peaks (Ahmadi and Hosseini 2013):

$$D = \frac{0.9 \lambda }{{\beta {\text{Cos}} \left( \theta \right)}},$$
(2)
Fig. 2
figure 2

Standard XRD pattern of the a pure Fe3O4 nanoparticles (JCPDS card no. 96-900-5815), b pure Fe2O3 nanoparticles (JCPDS card no. 96-101-1241), and XRD pattern of synthesized c naked, d \({\text{L}}_{{{\text{cysteine}}}} { - }\)coated \({\text{ Fe}}_{{3}} {\text{O}}_{{4}}\) nanoparticles, and e \({\text{Fe}}_{{3}} {\text{O}}_{{4}}\)-\({\text{L}}_{{{\text{cysteine}}}}\)-GQDs nanocomposites of the dried powder sample at room temperature

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where D is the mean crystalline size, l is the X-ray wavelength, b is the FWHM (radian) of diffraction peak, and \(\theta\) is diffraction angle. According to the formula and diffraction patterns, the average crystal size of the Fe3O4 particles was calculated by about 7 nm. Figure 2d, e confirms that the presence of LCysteine and GQDs did not considerably alter XRD patterns of the bare MNPs, so the nature and phase structure of the MNPs remains constant; however, it could be seen that some insignificant peaks with low intensity appear in the functionalized sample. The presence of GQDs can change some of the iron oxide nanoparticles’ structure from Fe3O4 to Fe2O3. As a result, another peak at 33.35°, which is related to the (104) appears and confirms the formation of a Fe2O3.

Fourier-transform infrared (FT-IR) spectroscopy

The FT-IR spectra and characterization bands of Fe3O4–LCysteine–GQDs nanocomposites compare to the naked Fe3O4 nanoparticles’ spectra, pure LCysteine, and Fe3O4–LCysteine are shown in Fig. 3. Figure 3a shows the FT-IR spectra of uncoated Fe3O4 nanoparticles. The broad peak at 3423.40 cm−1 attributes to the presence of stretching and bending vibrations of the O–H bond and this is due to the entrapped moisture and adsorbed water. The two peaks at 572.82 cm−1 and 632.60 cm−1 are assigned to iron oxide nanoparticles’ Fe–O bond vibration. The other peaks are related to the alkyl stretching modes, which were used during the synthesis procedure for pH rise. Figure 3b represents the \({\text{L}}_{{{\text{Cysteine}}}}\) FT-IR spectra. Typically, most of the amino acids can exist as zwitterions, exhibiting both carboxylic (-COOH) and amino (-NH2) functional groups (Sangeetha and Philip 2013). As mentioned above, LCysteine is a neutral amino acid, so the FT-IR spectrum of the commercial LCysteine shows broad characteristics range band of NH3+ stretching mode at 3000 cm−1, and the N–H bond of the amino group is observed at 1501.05 cm−1. The presence of the thiol group is virtually confirmed with the band at 2069.31 cm−1 (Hai et al. 2018a, b; Tiwari 2017). LCysteine coated Fe3O4 nanoparticles show characteristic bands in FT-IR spectroscopy in Fig. 3c. These spectra exhibit insignificant changes in characteristic bonds compared with pristine LCysteine, shown in Table 1. The absence of the S–H band at 2069.31 cm−1 strongly shows that magnetite nanoparticles have been linked with the thiol group of LCysteine to form a core–shell structure (Cohen 2008; Tiwari 2017; Karamipour 2015). The Fe–S peak cannot be seen in Fig. 3c, since Fe–S stretching, and bending peaks appear below 400 cm−1 (Lauterbach 2015). Finally, the FT-IR spectra of the final nanocomposite are depicted in Fig. 3d. A decrease in N–H characteristic band intensity can result in forming the bond between the amino acid functional group of LCysteine and GQDs (Hasanzadeh 2016; Mohammad-rezaei 2014). The wide peak at 3000 cm−1 for the samples, including LCysteine is related to the NH3+ stretching mode. The other characteristic bands with their wavenumber are shown in Table 1.

Fig. 3
figure 3

Typical FT-IR spectra of the powder sample of a naked magnetite nanoparticles, b pure \({\text{L}}_{{{\text{cysteine}}}}\), c \({\text{L}}_{{{\text{cysteine}}}} { - }\)coated \({\text{Fe}}_{{3}} {\text{O}}_{{4}}\) nanoparticles, and d \({\text{Fe}}_{{3}} {\text{O}}_{{4}}\)-\({\text{L}}_{{{\text{cysteine}}}}\)-GQDs nanocomposites at room temperature from 4000–400 cm−1

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Table 1 The list of characteristic band wavenumber of LCysteine, LCysteine coated MNPs, and Fe3O4-LCysteine-GQDs
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X-ray photoelectron spectroscopy (XPS) analysis

XPS measurement leads to a deeper understanding of the nature of the bonding types between LCysteine and Fe3O4 nanoparticles. The results of the XPS analysis of LCysteine functionalized Fe3O4 is shown in Fig. 4a, d. There are 5 distinct, sharp peaks at 710 eV, 531 eV, 401 eV, 288 eV, and 163 eV in the full spectrum, which are described as the Fe2p, O1s, N1s, C1s, and S2p, respectively (Fig. 4a). The possibility of forming the bonding of amino and carboxylic groups of LCysteine to the Fe3O4 nanoparticles can be neglected, since the singlet spectrum of N1s and O1s in the LCysteine functionalized Fe3O4 is detected, which are correlated to the amino and carboxylic groups of the LCysteine molecules. In the C1s spectrum in Fig. 4b, three peaks, which are detected at 288.2 eV, 287.6 eV, and 285.1 eV, ascribe the presence of C–C, C–N/C–H/C–OH, and C = O, respectively and show the existence of LCysteine in the final product. These spectra also announce that the carboxyl group in LCysteine molecules did not react with the iron oxide nanoparticles. Therefore, according to the most similar research, the most probable binding type is the formation of the Fe–S bond (Ahmadi 2011; Sangeetha and Philip 2013; Cohen 2008). The S2p XPS spectra of MNPs functionalized with LCysteine is observed in Fig. 4c. Instead of two peaks at 163.7 eV and 164.8 eV for pure LCysteine, other characteristic peaks appear at 161.2 eV and 162.8 eV, Which confirm the chemical bond of Fe–S in the core–shell structure and assure us that the LCysteine interacts with Fe3O4 nanoparticles via its thiol group (Cohen 2008). The peaks at 709.5 eV and 722.9 eV reveal the Fe2p spectrum for the functionalized MNPs and associate to the MNPs chemical bonds (Fig. 4d). Besides, compared with the bare MNPs, a doublet peak is detected at 717.19 eV and 731.4 eV when the particles coated with LCysteine. The presence of 2 additional peaks at Fe2p XPS spectra of core–shell structure compare to the bare MNPs is due to the doublet nature of the excited ions, where the final state is available with two possible energies, formed from a set of degenerated states. These states of energies depend on the total angular momentum quantum number j = l + s, where l is the orbital angular momentum number and s is the spin angular momentum quantum number.

Fig. 4
figure 4

XPS spectra of \({\text{L}}_{{{\text{cysteine}}}} { - }\)coated \({\text{Fe}}_{{3}} {\text{O}}_{{4}} { }\) nanoparticles of a overview, and the high-resolution spectra of b C1s, c S2p, and d Fe2p with a monochromatic Al \(K_{\alpha }\) X-ray source using chamber pressures of \(10^{ - 8}\) Pa at room temperature

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Scanning electron microscopy (SEM) characterization

SEM images of Fe3O4, Fe3O4–LCysteine, and Fe3O4–LCysteine–GQDs nanostructures, shown in Fig. 5a–c, illustrate the homogenized distribution and sponge-like morphology of all samples in which the mean size of the particles is about 15–20 nm. Although the excellent stability and high dispersion of nanoparticles of magnetite in an aqueous solution, they aggregate as soon as they dry. Therefore, in this micrograph, each cluster has a number of smaller particles of magnetic nanoparticles. Furthermore, EDX measurements were used to confirm the composition of the synthesized samples (Fig. 5d–f). The quantity of the Fe, O, C, N, and S contents are shown in Table 2. The calculated elements’ content quantities reveal the successful deposition of the MNPs–LCysteine–GQDs nanocomposites.

Fig. 5
figure 5

FESEM micrograph of a naked, b Lcysteine-coated Fe3O4 nanoparticles, c Fe3O4-Lcysteine-GQDs nanocomposites of a dried sample which are coated with Pt and particle size range from 15 to 20 nm, and EDX spectra of d naked, e Lcysteine-coated Fe3O4 nanoparticles, f Fe3O4-Lcysteine-GQDs nanocomposites in a vacuum atmosphere

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Table 2 Elements’ content quantities of \({\text{Fe}}_{{3}} {\text{O}}_{{4}}\), \({\text{L}}_{{{\text{cysteine}}}} { - }\)coated \(Fe_{3} O_{4}\) nanoparticles and \({\text{Fe}}_{{3}} {\text{O}}_{{4}}\)-\({\text{L}}_{{{\text{Cysteine}}}}\)-\({\text{ GQDs}}\) nanocomposite measured by EDX spectra
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Transmission electron microscopy (TEM) characterization

TEM images in Fig. 6a refer to the naked Fe3O4 in the absence of LCysteine, and Fig. 6b is related to the modified Fe3O4 with LCysteine. As shown in Fig. 6a, bare iron oxide particles have a high tendency to aggregate due to their strong magnetic and long-range Vander Waal’s forces. Figure 6b shows Fe3O4 particles functionalized with LCysteine and its dispersion behavior. The core size of the particles has been increased during the functionalization process. It is clear from size distribution histogram diagrams in Fig. 6c, d, the mean particle size for Fe3O4 nanoparticles, Fe3O4–LCysteine core–shell structure, and LCysteine layer thickness are about 7.5 \(\pm\) 3 nm, 11 ± 4 nm, and 3 ± 1 nm, respectively.

Fig. 6
figure 6

TEM micrographs of a naked, b \({\text{L}}_{{{\text{cysteine}}}}\)-coated \({\text{Fe}}_{{3}} {\text{O}}_{{4}}\) nanoparticles, and with their corresponding particle size distribution of c naked, d \({\text{L}}_{{{\text{cysteine}}}} { - }\)coated \({\text{Fe}}_{{3}} {\text{O}}_{{4}}\) nanoparticles with 7.5 \(\pm\) 3 nm, and 11 \(\pm\) 4 nm, respectively

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Vibrating sample magnetometer (VSM) diagrams

Magnetic hysteresis curves of the as-prepared Fe3O4, Fe3O4–LCysteine, and Fe3O4–LCysteine–GQDs structures at room temperature are depicted in Fig. 7a–c. The values of saturation magnetization (Ms), remnant magnetization (Mr), and magnetic coercivity (Hc) for all of the prepared samples show a superparamagnetic behavior and reveal their small size (Table 3). As it can be observed from data in Table 3, the superparamagnetic behavior of the bare Fe3O4 nanoparticles is confirmed from the high Ms and low Hc, which are 56.5 emu. g−1 and 0.28 Oe, respectively. Additionally, Ms decreases to 53.3 emu. g−1 when LCysteine is coated MNPs, while Hc merely increases and reaches to 0.38 Oe. In other words, due to the core–shell interface and shielding effects, the LCysteine layer decreases the magnetic properties compared with the bare magnetite nanoparticles. Finally, magnetic saturation for the Fe3O4–LCysteine–GQDs nanocomposite decreases to 28.99 emu. g−1, while the Hc reaches 0.09 Oe. Although the presence of GQDs decreases the Ms, the nanostructure still shows superparamagnetic properties. The reduction in Ms value has been referred to the disordering of the magnetic moment. The higher the GQDs shell thickness onto the magnetite nanoparticles leads to the lower magnetization saturation. Additionally, according to the XRD analysis, the change in the structure of Fe3O4–Fe2O3 is another reason to cause a decrease in the Ms value. As a result of the nanocomposite’s magnetic–nonmagnetic hybridization, these results are still sufficient for MRI contrast agents.

Fig. 7
figure 7

Magnetic hysteresis curve of a bare \({\text{Fe}}_{{3}} {\text{O}}_{{4}}\) nanoparticle b \({\text{L}}_{{{\text{cysteine}}}}\) coated \(Fe_{3} O_{4}\) and c \({\text{Fe}}_{{3}} {\text{O}}_{{4}}\)-\({\text{L}}_{{{\text{cysteine}}}}\)-GQDs nanocomposites at room temperature. These nanostructures exhibit superparamagnetic behavior with high Ms, and low Hc

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Table 3 Magnetic properties of \(Fe_{3} O_{4}\), \({\text{L}}_{{{\text{cysteine}}}}\)-coated \({\text{Fe}}_{{3}} {\text{O}}_{{4}}\) nanoparticles and \({\text{Fe}}_{{3}} {\text{O}}_{{4}}\)-\({\text{L}}_{{{\text{Cysteine}}}}\)-\({\text{ GQDs}}\) nanocomposite
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Optical study

The UV–Vis spectra of GQDs and MNPs covered with GQDs lead to a strong absorption peak at 276 nm and 270 nm with a tail extending to the visible region, respectively (Fig. 8a, b). The high UV–Vis adsorption is due to the presence of the GQDs, which leads to the surface sphere and surface cavity mode at the interface of MNPs and GQDs. Photoluminescence (PL) spectra of the GQDs, MNPs, and Fe3O4–LCysteine–GQDs nanocomposites are recorded at room temperature (Fig. 8c–e). A detailed PL study for GQDs was carried out using different excitation wavelengths. Apart from most of the carbon-based nanomaterials with highly dependent PL excitation wavelength, GQDs show a different behavior (Mohammad-rezaei 2014; Li 2010). As it can be seen in Fig. 8c, the emission wavelength of GQDs is nearly excitation-independent (Zhuo et al. 2012). By getting deeper in Fig. 8c, it can be observed that as the excitation wavelength varies from 235 to 400 nm, GQDs emit a stable, strong PL peak at about 460 nm (2.69 eV). This phenomenon happens when the size and sp2 clusters on the surface of the GQDs remain constant and uniform (Dong 2012). Besides, the PL spectra of the bare MNPs revealed a stable emission at 530 nm (2.34 eV) when it is excited at 235 nm (Fig. 8d). The distinct PL features and different emission wavelengths in the visible range were measured with photoexcitation at 235 nm for the final product, which presents an asymmetric emission curve (Fig. 8d). Figure 8d shows a broad emission peak at around the visible range with two unequivocal adsorption peaks at about 460 nm and 530 nm. It is clear that the former peak nearly stands at the same position compare with the pristine GQDs, while the second peak can be ascribed to the presence of magnetic core in the final nanocomposites. As a result, the PL spectra of the final product shows a stable emission in the visible range, which makes this nanocomposite a promising candidate for bioimaging applications.

Fig. 8
figure 8

The UV–Vis spectra of a as prepared \({\text{ GQDs}}\), and b \({\text{Fe}}_{{3}} {\text{O}}_{{4}}\)-\({\text{L}}_{{{\text{Cysteine}}}}\)-\({\text{ GQDs}}\) nanostructure from 250 to 800 nm and photoluminescence spectra of c \({\text{ GQDs}}\) at different excitation wavelength ranging from 235 to 400 nm reveals a stable emission at 465 nm, d magnetite nanoparticles at 235 nm excitation wavelength, which shows emission at 530 nm, and e \(Fe_{3} O_{4}\)-\({\text{L}}_{{{\text{Cysteine}}}}\)-\({\text{ GQDs}}\) at room temperature at 235 nm excitation wavelength, which shows stable emission in the visible range at 460 nm and 530 nm

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Conclusion

In this research, Magnetite nanoparticles were synthesized via a co-precipitation method and were coated with LCysteine to produce a core–shell structure, while GQDs were obtained by hydrothermal approaches. Finally, a facile, two-step method was developed to synthesize Fe3O4–Lcysteine–GQDs nanocomposite. In this nanocomposite, LCysteine acts as a linker between MNPs and GQDs and improves the MNPs’ biocompatibility, and protects them from excessive oxidation and agglomeration. XRD analysis confirms the formation of the synthesized MNPs, LCysteine coated MNPs, and MNPs–LCysteine–GQDs nanostructure. The XPS and FT-IR measurements reveal the formation of Fe–S bond between MNPs and LCysteine, so LCysteine molecules bind to the MNPs via its thiol groups. SEM and TEM images show that MNPs have a high tendency to agglomerate and exhibit sponge-like morphologies with 7.5 ± 3 nm magnetic core. Additionally, EDX results show the weight percent of different elements in the samples. VSM diagram reveals superparamagnetic behavior with high Ms and negligible Hc for the final product. Investigation of optical properties from the UV–Vis and PL spectra confirm that the final nanocomposite reveals a desirable photoluminescence behavior in the visible range. These results show that the obtained nanocomposite reveals proper magnetic characteristics and stable optical properties simultaneously. According to the conclusion mentioned above, the synthesized hybrid nanostructure reveals a suitable emission in the visible range and can be used in biosensors for detecting different cancer cells and viruses like breast cancers, COVID-19, and HIV. In addition, due to the final composite’s superparamagnetic behavior, this nanostructure can also be used as an MRI contrast agent. In addition, increasing outcomes with magnetic nanostructures in recent in vivo research suggest the huge potential for cancer diagnostic and therapy. In recent years, treating and sensing molecules, cancer cells, and viruses are multi-billion-dollar industries among academia, clinicians, and biomedical engineers.